Some systems require a constant power supply and therefore provide for a battery backup in case the main power supply fails. Conventional solutions for switching control are either diode or comparator-based. In a diode-based system, a pair of PN diodes connect the anodes to the supply (+) pins and the common cathode supply the client domain. Comparator-based systems are based on standard fast comparators. The comparator senses the values of the supply voltages, compares them to a reference voltage and further controls the gates of the power switches. The comparators must be able to deal with input common mode ranges that match the supply voltage ranges. Typically, supply ranges in microcontroller systems are between 1.8 and 5.5V.
The drawbacks of the diode solution are, although it would be very area-effective, the DC voltage drop, of about 0.7V, would critically affect the supply range of the overall system. This makes it unsuitable for certain applications because the minimum VDD would increase from, for example, 1.8V to 2.5V.
In a comparator-based system, a lower the filtering capacitance requires a higher comparator speed requirement. In order to be sufficiently fast, the comparators need high bias currents, consequently causing high power consumption. Comparator-based systems also need a large silicon area. If a reference voltage must be provided by a separate circuit, even more area is needed.